1. Field of the Invention
This invention relates to electrochemical cell stacks and electrodes used therein.
2. Description of the Prior Art
A fuel cell is an electrochemical cell. A typical fuel cell is comprised of a matrix material for holding electrolyte and an electrode disposed on each side of the matrix and in contact therewith. Reactant gases are fed to the nonelectrolyte facing sides of each electrode. In a stack of fuel cells separator plates are disposed between adjacent cells. The plates have ribs formed on each side thereof. The ribs abut the electrodes of adjacent cells and form U-shaped channels behind the electrodes. Reactant gas is carried to the electrodes via these channels. FIGS. 1 and 2 of commonly owned U.S. Pat. No. 3,994,748 and FIG. 2 of commonly owned U.S. Pat. No. 3,990,913 show fuel cell stack constructions in accordance with the foregoing description. In FIG. 2 of U.S. Pat. No. 3,994,748 it can be seen that each electrode comprises a thin catalyst layer disposed on a somewhat thicker substrate or support layer. The catalyst layer is in direct contact with the matrix, and the substrate contacts the separator plate ribs.
It is known in the art that the electrolyte liquid volume varies depending upon the operating mode of the fuel cell. When the fuel cell is of the type which holds its electrolyte trapped within a matrix, provision must be made for accommodating excess electrolyte volume beyond that which the matrix is capable of holding. A well known solution to this problem is the use of what is called a reservoir layer disposed behind one or both of the electrodes. The reservoir layer acts as a sponge to store excess electrolyte. Wicking paths from the matrix through the electrode to the reservoir layer are typically provided to permit transfer of the electrolyte therebetween as the liquid volume increases and decreases. Commonly owned U.S. Pat. Nos. 3,779,811 and 3,905,832 describe this type of approach to handling liquid volume changes in fuel cells.
Separate reservoir layers and ribs in the separator plate are undesirable from both an economic and technical point of view. For example, separator plates are usually made from graphite or a graphite composite and must be gas impermeable; it is expensive putting ribs in such a material. With regard to separate reservoir layers, additional components in a cell package always means increased fabrication and materials costs. Additionally, when separate reservoir layers are used the reactant gas, in order to reach the catalyst layer, must pass through both the partially filled (i.e., with electrolyte) reservoir layer and the electrode substrate. Usually holes or wetproofed areas are required in the reservoir layers to reduce diffusion losses; however, this increases the cost of fabricating the reservoir layers. The thickness of the reservoir layers must also be kept to a minimum for the purpose of reducing these diffusion losses, despite the fact that this restricts the amount of excess electrolyte which can be handled by the cell.